a(n) = a(n-1) + 3*a(n-2) - 2*a(n-3) - 2*a(n-4), where a(0) = 1, a(1) = 2, a(2) = 3, a(3) = 4.

Terms

a(0) =1a(1) =2a(2) =3a(3) =4a(4) =7a(5) =9a(6) =16a(7) =21a(8) =37a(9) =50a(10) =87a(11) =121a(12) =208a(13) =297a(14) =505a(15) =738a(16) =1243a(17) =1853a(18) =3096a(19) =4693a(20) =7789a(21) =11970a(22) =19759a(23) =30705a(24) =50464a(25) =79121a(26) =129585a(27) =204610a(28) =334195a(29) =530613